Modulation of Nickel Pyridinedicarboxamidate Complexes to Explore the Properties of High‐valent Oxidants

Pirovano, Paolo; Twamley, Brendan; McDonald, Aidan R.

doi:10.1002/chem.201704618

Cited by 17 publications

(26 citation statements)

References 69 publications

(58 reference statements)

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“…This contrasts with the previously reported [Ni III (OAc)(L*)] complex which was thermally stable at 25 °C when prepared with NaOCl/AcOH . The instability may be attributed to ligand oxidation (at the isopropyl group) which we previously observed for L, but not for L*. ,, …”

Section: Resultscontrasting

confidence: 92%

“…The τ 4 parameters for 1a and 2a were 0.18 and 0.26, respectively, which indicated that the complexes were in slightly distorted square planar geometries (τ 4 = 0 for pure square planar, τ 4 = 1 for pure tetrahedral) . The geometric parameters of 1a and 2a were similar to those of previously reported [Ni II (OX)(L*)] − (OX = OH, OCO 2 H, OAc, ONO 2 ) and [Cu II (OH)(L)] − complexes. ,,,, …”

Section: Resultssupporting

confidence: 79%

“… The red shift in the higher-energy band of 1b is presumably a result of the different substituents on the aryl amidate donors (L = 2,6-diisopropylphenyl; L* = 2,6-dimethylphenyl). Such features are a hallmark of Ni III complexes supported by 2,6-pyridinedicarboxamidate donors and are also generally typical of Ni III complexes. − ,− Compound 2b also displayed two intense bands in the visible and NIR regions of its electronic absorption spectrum at λ max = 630, 805 nm (Figure ). Tolman’s [Cu III (OH)(LX)] complexes displayed a single intense visible band at λ max = 540 nm, − whereas 2b showed two absorption bands at lower energies, presumably as a result of the weaker-field donor ancillary ligand (OAc for 2b , versus OH in [Cu III (OH)(LX)]).…”

Section: Resultsmentioning

confidence: 91%

“…28 The geometric parameters of 1a and 2a were similar to those of previously reported [Ni II (OX)(L*)] − (OX = OH, OCO 2 H, OAc, ONO 2 ) and [Cu II (OH)(L)] − complexes. 22,23,25,27,29 Preparation and Characterization of 1b and 2b. [Ni III (OAc)(L)] and [Cu III (OAc)(L)] (1b and 2b) were generated using the one-electron oxidant magic blue (tris(4bromophenyl)ammoniumyl hexachloroantimonate) as an oxidant.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…We recently reported the preparation and characterization of a family of terminal Ni III –oxygen adducts (Scheme ), [Ni III (OX)(L*)] (OX = OCO 2 H, O 2 CCH 3 (OAc), and ONO 2 ), supported by N , N′ -(2,6-dimethylphenyl)-2,6-pyridinedicarboxamidate (L*). − We observed a 20-fold enhancement in the rate constant for HAT oxidation upon exchanging ONO 2 for OAc. Having gained an understanding of the anionic ancillary ligand (OX in [Ni III (OX)(L*)]) in HAT oxidations, and herein we explore the role of the metal ion, d 7 Ni III versus d 8 Cu III , in driving PCET oxidation of a variety of substrates.…”

Section: Introductionmentioning

confidence: 99%

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High-Valent d⁷ Ni^III versus d⁸ Cu^III Oxidants in PCET

et al. 2019

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Oxygenases have been postulated to utilize d 4 Fe IV and d 8 Cu III oxidants in proton-coupled electron transfer (PCET) hydrocarbon oxidation. In order to explore the influence the metal ion and d-electron count can hold over the PCET reactivity, two metastable high-valent metal−oxygen adducts, [Ni III (OAc)(L)] (1b) and [Cu III (OAc)(L)] (2b), L = N,N′-(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamidate, were prepared from their low-valent precursors [Ni II (OAc)-(L)] − (1a) and [Cu II (OAc)(L)] − (2a).The complexes 1a/ b−2a/b were characterized using nuclear magnetic resonance, Fourier transform infrared, electron paramagnetic resonance, Xray diffraction, and absorption spectroscopies and mass spectrometry. Both complexes were capable of activating substrates through a concerted PCET mechanism (hydrogen atom transfer, HAT, or concerted proton and electron transfer, CPET). The reactivity of 1b and 2b toward a series of para-substituted 2,6-di-tert-butylphenols (p-X-2,6-DTBP; X = OCH 3 , C(CH 3 ) 3 , CH 3 , H, Br, CN, NO 2 ) was studied, showing similar rates of reaction for both complexes. In the oxidation of xanthene, the d 8 Cu III oxidant displayed a small increase in the rate constant compared to that of the d 7 Ni III oxidant. The d 8 Cu III oxidant was capable of oxidizing a large family of hydrocarbon substrates with bond dissociation enthalpy (BDE C−H ) values up to 90 kcal/mol. It was previously observed that exchanging the ancillary anionic donor ligand in such complexes resulted in a 20-fold enhancement in the rate constant, an observation that is further enforced by comparison of 1b and 2b to the literature precedents. In contrast, we observed only minor differences in the rate constants upon comparing 1b to 2b. It was thus concluded that in this case the metal ion has a minor impact, while the ancillary donor ligand yields more kinetic control over HAT/CPET oxidation.

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Section: Resultscontrasting

confidence: 92%

Section: Resultssupporting

confidence: 79%

Section: Resultsmentioning

confidence: 91%

Section: ■ Results and Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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High-Valent d⁷ Ni^III versus d⁸ Cu^III Oxidants in PCET

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Hydrocarbon Oxidation by a Porphyrin‐π‐Cation Radical Complex

2023

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Heme and chlorin π‐cation radical oxidants are widely implicated in biological and synthetic oxidation catalysis. Little insight into the role of π‐cation radicals in proton coupled electron transfer (PCET) oxidation is available. We prepared a NiII‐porphyrin‐π‐cation complex ([NiII(P⋅+)]) and found it to be capable of the oxidation of a variety of simple hydrocarbon substrates. Interestingly, some of the products were hydroxylated, with ([NiII(P⋅+)]) working in concert with atmospheric O2 to yield hydroxylated hydrocarbons. Kinetic data suggested that the porphyrin‐π‐cation radical species oxidised substrates through a concerted PCET mechanism, where the porphyrin‐π‐cation radical accepted the electron, and the proton was transferred to a free anion. Our findings highlight the potential role of π‐cation radicals as hydrocarbon activators, demonstrating that porphyrin ligand non‐innocence could be a readily manipulated resource for oxidation catalyst development.

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